Stocked up on guns and ammo, which not necessarily is of good quality early game doesn't mean much. I once looted a T1 and T3 iron axe and they have a somewhat huge difference in damage. Imagine between guns, where the base damage is higher, you'll have bigger differences. And yes, Primitive Bow (let's call basic weapon) can be crafted from basic materials, only Wooden Bow (let's call it intermediate) and Compound Bow (let's all it advanced) need materials, each of their own.
I think i know what the gist is of the topic here. In short: OPTIONS
Firstly, i would like to address something that noone really mentioned. I would love to have the option to choose the crafting quality i want to make. For example, i could be able to craft T4 Wooden Bow, but gathered enough for a T3. Now i NEED that bow, so it's better for me to craft a T3 and gathering materials for a T4 would take too much time. Another example, i could be making T2, but prefer to make T1 and leave the rest of the materials for a T3 or T4 later on. I know tiers are supposed to work like "yeah, i know how to do it better", but considering it also needs more materials, it should have an option to choose what tier you want to craft.
Secondly, something for mid to late game. ALTERNATIVES. As much as i like being dependant on RNG, it's always better to have options. I'm pretty sure the trader and looting seems like enough and perhaps this suggestion is something more of an end game. Why not introduce some kind of special workbench, machine or perk that would be able to craft these parts? Considering we are able to craft intricate items as switches, dart traps, various sensors, turrets, doors, hatches and draw bridges, as well many others, there is no sense we are not able to make such small "items" for craft only a handful of weapons.
From technological standpoint, you should be able to craft practically anything, from gameplay standpoint not necessarily. So, second suggestion: Add a way to craft "parts" in some difficult, not easy to obtain way, requiring a couple steps to get/produce or simply introduce a mechanic for "requesting" some materials at a trader. Like, you know you want specifically bow parts and after the shop refreshes it allows you to buy some bow parts, along with low level bows.